Court-authorized access to telephone communications is an important tool for effective law enforcement. The introduction of new, digitally-based technologies, transmission modes, services, and features have made it increasingly difficult for law enforcement to conduct court-authorized electronic surveillance. Tapping calls in packet-based communication networks is particularly difficult because communication channels may be dynamically provisioned, such that there is no static line to tap, and because subscribers frequently move from one service provider to another. In packet-based communications networks, a media gateway is a network entity that switches media stream communications between its input and output ports and may translate the media stream from one format to another format. Media gateways do not participate in call signaling and consequently do not maintain call state information. A media gateway controller performs signaling functions to establish and tear down calls across media gateways.
One problem with lawfully intercepting communications in conventional media gateways is that the protocols used to control connections in media gateways fail to provide an easily scalable mechanism for intercepting a media stream. For example, the MEGACO protocol, as described in ITU-T Recommendation H.248, includes commands for establishing and modifying contexts in a media gateway. A context is analogous to a call or a communications session between one or more parties. In a media gateway, a context is defined by its endpoints, referred to as terminations. Typically, a context has two terminations in the media gateway—one for communications to and from one party and one for communications to and from another party. The context also defines the direction of the communication, or media stream, between the terminations.
FIG. 1 is a block diagram of a call context 100 having a lawful intercept through a media gateway modeled after a traditional three-way calling configuration. In FIG. 1, the call context includes a bidirectional media stream 102 between Termination A 104 and Termination B 106 in a media gateway 107. The media stream 102 carries media traffic between Termination A 104 and Termination B 106 and allows end users to communicate normally. A third termination, Intercept A 108, is added to the context to accommodate authorized monitoring of the media stream 102 by law enforcement. A TDM matrix 109 switches the media stream 102, 110 between the terminations. Normally, in a three-way call, the media stream between each termination is bi-directional. However, it is desirable for the media stream 110 toward Intercept A 108 to be one-way to prevent sound that might lead to the detection of Intercept A from being received by the other terminations. Thus, the media stream 110 toward Intercept A 108 is configured as a one way only stream to allow law enforcement to monitor the call without interference.
The MEGACO protocol provides for a topology descriptor, which is used to specify flow directions between terminations in a context. The default topology of a context is that each termination's transmission is received by all other terminations. Changing the association between terminations changes the topology of a context. Thus, to implement a lawful intercept using the topology descriptor, the lawful intercept media streams should be configured as one-way toward the law enforcement monitoring termination.
A shortcoming of this arrangement becomes apparent if more than one law enforcement agency is intercepting the media stream. As shown in FIG. 2, three intercept terminations 108 have been added to the call context to accommodate interception by three different agencies. Changes made to the call impact each termination in the context. For example, if the end user on Termination A 104 wishes to add another termination to establish a three-way conversation or if one of the law enforcement agencies wishes to cease its interception of the call, the topology descriptor in the call context would need to be updated, which involves several steps of MEGACO commands. This can be a cumbersome management task and may result in delays in establishing or terminating calls. Moreover, the media gateway may limit the number of terminations permitted in a call context. The limitation on terminations may limit the number of lawful intercepts that could be applied and may altogether prevent the application of a lawful intercept.
Accordingly, there is a need to provide a mechanism to permit the lawful intercept of a call while minimizing the increased load on call management resources.